Our primary objective is to determine the molecular mechanism(s) that control synthesis of bacterial proteins in response to availability of nitrogen. We have studied pleiotropic mutant strains of Salmonella typhimurium (glnF and glnR) that are unable to regulate synthesis of glutamine synthetase or amino acid transport components in response to availability of nitrogen. Based on analysis of these mutant strains our working model for nitrogen control is that the glnF product catalyzes synthesis of a low molecular weight signal of nitrogen limitation and the glnR product is the macromolecular receptor for the signal, which functions directly as a regulator of transcription. We propose to test this model in several ways: (1) by demonstrating effects of the glnR and glnF products on synthesis of glutamine synthetase in an in vitro protein synthesizing system (coupled transcription-translation system). Effects of these products in vitro will provide an assay for identifying and purifying them, (2) by isolating and analyzing additional regulatory mutant strains, particulary strains that would be predicted if the model is correct. These include strains with altered glnR products and altered cis-acting regulatory sites adjacent to glnA, the structural gene for glutamine synthetase, (3) by determining whether differences in the extent of nitrogen control among the enteric bacteria can be accounted for by differences in the glnR product (Based on intergeneric complementation tests). Our second objective is to assess the significance of reversible covalent modification of glutamine synthetase in vivo. We will do comparative physiological studies of wild-type and mutant strains in which covalent modification is altered.